All of the part-time workers are going to celebrate the owner’s 77th birthday in his convenience store.
You can rejoice because your lifespan has reached this special point. That special point is your seventy seventh birthday. So many sevens, lucky!
The readings are the on'yomi reading. You've seen the on'yomi reading of 喜 before, though it may be hard to remember at this point. Maybe you can think about how when you reach this seventy seventh birthday, all you want is a key (き) to the city so you can do whatever you want.